1. Field of the Invention
The present invention pertains generally to wells for production of petroleum products and more specifically concerns wells located in a subsea environment where the pressure containing integrity of wells is of particular concern from the standpoint of environmental protection and for protection of workers and equipment from the hazards of pressure leakage from wells. More particularly, the present invention provides a non-intrusive method for monitoring pressure in well casing annuli without compromising the pressure containing integrity of the well system in any way, and thus permitting excessive pressure in typically inaccessible annuli to be detected, and corrective actions taken before a hazardous event can occur that might impact human life, the environment or property.
2. Description of the Prior Art
While the present invention has application to petroleum producing wells other than subsea well systems, for purposes of simplicity and to facilitate ready understanding of the invention by others, the present invention is described herein particularly as it relates to subsea wells.
The Minerals Management Service (MMS) recently revised its policy on Sustained Casinghead Pressure (SCP) for the Gulf of Mexico Outer Continental Shelf Region (GOMR). The MMS issued a proposed Notice to Lessees and Operators (NTL) to define changes that are forthcoming to its current policy. Current (previous) policy is defined in a Jan. 13, 1994 Letter to Lessees (LTL).
SCP occurs when one or more leaks develop in the barriers designed to achieve and maintain pressure control of wells. SCP is defined as:
1. A pressure measurable at the casinghead of a casing annulus that rebuilds when bled down;
2. A pressure that is not due solely to temperature fluctuations; and
3. A pressure that has not been deliberately applied.
It is thus considered desirable to monitor all casing annuli for SCP on all subsea trees to ensure early detection of pressure buildup in any of the various annuli thereof.
The Jan. 13, 1994 LTL required all annuli on offshore producing wells to be monitored for SCP. However, this regulation is written primarily for wells on conventional, fixed platforms and departures have been granted for subsea wells. The accepted requirement for subsea wells is to monitor only the annulus between the production tubing and production casing strings (the xe2x80x9cAxe2x80x9d annulus) since it can be monitored by pressure sensing lines passing through the wellhead, without any need for penetrating the outer pressure containing housing or wall which isolates annulus pressure from the seawater or other environment. The conventional method for monitoring the xe2x80x9cAxe2x80x9d annulus is to provide an annulus monitor line in the tree""s production control umbilical and/or to provide an electronic pressure sensor in the tree""s annulus flowpath. The control line and/or pressure sensor can be isolated from the production annulus of the well by one or more valve closures on the subsea tree. Wells with SCP in the xe2x80x9cAxe2x80x9d annulus that is less than 20% of the minimum internal yield pressure (MIYP) of the affected casing can be produced on a xe2x80x9cself approvedxe2x80x9d basis, provided the annulus pressure can be bled to zero through a xc2xdxe2x80x3 needle valve in 24 hours or less. Criteria is also established to determine unsustained casing pressure that is typically caused by thermal Ad effects during well start up.
Surface wellhead systems, used on land and on offshore platforms, provide pressure containing side outlets in the casing and tubing heads, from which annulus pressure can be monitored. API Specification 17D does not permit body penetrations in high pressure subsea wellhead housings. Even if penetrations were allowed in subsea wellhead, housings, the overall safety of the well would be at higher-risk because each wellhead penetration creates a potential leak point. Obviously when a wellhead is located at or near the seabed leakage or a body penetration connection would be difficult to detect until a major problem has occurred.
In 1995, a laboratory demonstration was provided for a non-intrusive wellhead casing monitoring system to the Deepstar Joint Industry Project. This non-intrusive annulus pressure monitoring system uses strain gauges on the outside of the wellhead housing. The elevation of the strain gauges on the wellhead corresponds to the annular areas between the casing hanger packoffs inside the wellhead housing. Pressure is monitored by correlating the strain measured on the outside of the wellhead housing to the pressure applied between the packoffs inside the wellhead housing. The strain gauge method has not progressed beyond the laboratory stage due to technical concerns about implementing the method for the subsea environment.
U.S. Pat. No. 5,544,707, dated Aug. 13, 1996, covers an adjustable seal sleeve mechanism that can be installed in the place of a normal packoff assembly on the production casing hanger to provide access to the annulus around the outside of the production casing (the xe2x80x9cBxe2x80x9d annulus). The position of the sleeve is adjusted mechanically by a running tool prior to installing the tree. When the tree is installed, pressure in the xe2x80x9cBxe2x80x9d annulus can be monitored separately from pressure in the production tubing annulus (the xe2x80x9cAxe2x80x9d annulus) through a side outlet in the tree body. Monitoring of the xe2x80x9cBxe2x80x9d annulus is achieved by conventional means, in the same manner as described above under current practice for the xe2x80x9cAxe2x80x9d annulus. The adjustable-sleeve approach only enables pressure to be monitored in the innermost two annuli of a well. Some subsea wells with extensive casing programs may have up to six annuli. The seals and ports on the adjustable sleeve are potential leak points that increase the overall safety risk for the well.
U.S. Pat. No. 4,887,672 covers a method that uses hydraulic couplers between the top of wellhead housing and the tree connector. The couplers enable ports in the wellhead and tree to communicate with each other when the tree is locked to the wellhead. A long vertical hole drilled from the coupler location in the top of the wellhead communicates with a short, internal, horizontal hole in the wellhead housing. The elevation of the internal hole exposes the annular area between casing hanger packoffs to the monitoring port. One coupler/port combination is used for each annulus to be monitored. The ports can be monitored through a line in the production umbilical and/or by an electronic pressure sensor, per current practice. The hydraulic coupler method is not believed to have been installed in the field. Orientation of the couplers prior to tree/wellhead makeup is critical and the couplers are subject to damage. Each port is a potential leak point that increases the overall safety risk for the well.
The Minerals and Management Service (MMS) of the U.S. Department of the Interior has proposed that wells with subsea trees will need to have all casing annuli monitored for sustained casing pressure, beginning with trees installed after Jan. 1, 2005. This requirement may present a safety risk to subsea wells, because the most straightforward method of accessing an annulus for pressure monitoring is to make a pressure containing penetration through the body of the pressure vessel. Since it is well known that all penetrations through the outer pressure containing housing of wellheads are potential leak points which add sealing risk, and thus safety risk, to the well system pressure monitoring in all well annuli will not be practical unless a safe system for doing so becomes commercially available. A further complication is that API Specification 17D for Subsea Wellhead and Christmas Tree Equipment explicitly prohibits body penetrations in high pressure subsea wellhead housings. Therefore, the recommended method for monitoring pressure in multiple annuli is by non-intrusive means, which does not exist according to current practice. It is to this need that the present invention is addressed.
The GOMR will not grant departures to allow pressure on the outside casings of subsea wells drilled or sidetracked after the effective date of the proposed NTL unless the lessee/operator can document in their Application for Permit to Drill (Form MMS 123) or Sundry Notice (Form MMS 124) that best cementing practices will be used. Proposed best cementing practices are defined by the MMS in Appendix B of the proposed NTL. This policy applies to all conductor, surface, intermediate and production casings. Pressure must be able to be detected at all times. For subsea wells, where only the production annulus can be monitored, diagnostics must be conducted as indicated in Appendix A of the proposed NTL, except that results for adjacent annuli will be restricted to monitoring tubing pressure response. That requirement is understood to mean that access must be provided to the xe2x80x9cAxe2x80x9d annulus as per current practice, and additional means must be provided to measure, but not bleed down or build up, the pressure in all outer annuli.
The objective for monitoring SCP on all annuli must be clearly established before a change in practice is implemented, to ensure that any change achieves the desired result. The implied objective is to eliminate safety hazards, and thereby avoid harm or damage to human life, the marine and coastal environment, and property. Therefore, the perceived advantages associated with monitoring SCP on all annuli must be achieved without increasing the risk or decreasing the reliability of current practice. Otherwise, well safety may be compromised rather than improved.
Before the proposed practice of monitoring SCP on/all casing annuli is implemented, concerns of safety, reliability and cost must be fully addressed. Wells are safe if pressures are known and controlled in a reliable manner.
There are two potential sources of SCP. The first source is from produced fluids coming out of the reservoir; the second is from formation pressure above the reservoir. If SCP results from produced fluids, due to a packer or tubing leak for example, it will be detected in the xe2x80x9cAxe2x80x2 annulus first. Current practice enables monitoring of SCP in the xe2x80x98Axe2x80x99 annulus, so the proposed practice of monitoring SCP in all casing annuli provides no additional benefit for the first source of SCP. If SCP results from formation pressure, the most likely causes are cement or structural failures. Rigorous implementation of properly engineered and designed cementing operations should minimize the risk of cement related failures. Universally accepted xe2x80x9cbest cementing practicesxe2x80x9d may come from the MMS, as described in Appendix B of the proposed NTL, or they may come from industry. Well casing programs and subsea wellhead equipment are structurally designed to control formation pressure in the outer casing annuli in a safe and reliable manner. Therefore, the need to monitor SCP in all casing annuli is questionable and should only be considered if a highly reliable means of achieving it can be established.
The reliability of any new SCP monitoring system should be equal to or better than current practice, otherwise, well safety may be compromised. The only methods that can be considered equally reliable to current practice are non-intrusive methods. Non-intrusive methods provide a means to monitor SCP without adding any new pressuring containing penetrations (intrusions) to the subsea wellhead housing or casing hanger systems. Every penetration is a potential leak point that decreases reliability. All intrusive methods add leak points, either externally through the wellhead housing or internally through movable seals on the casing hangers. Even though non-intrusive methods do not add leak points, their reliability at this point in time is unknown because non-intrusive methods are not fully developed and field proven. The reliability of the pressure data gathered by a non-intrusive system must be highly accurate, because the status of the well and important operational decisions will be based on the data acquired.
The cost associated with implementing a/multi-annulus pressure -monitoring system will depend on the method employed. Since the recommended method is a non-intrusive approach, and functional, field proven, non-intrusive methods do not exist at this time, the cost of implementation cannot be accurately estimated. However, the cost will be significant because wellhead systems, control systems and production umbilicals will all be impacted. The additional cost may preclude developing wells that are already considered economically marginal. For wells that are produced, a portion of the additional cost will have to be incurred during the drilling phase of a project, because the wellhead system will have to be equipped to interface with an SCP monitoring system.
The invention provides a non-intrusive method for monitoring pressure in well casing annuli. The pressure containing integrity of the well system is not compromised in any way. The overall safety of the well is enhanced because excessive pressure in a previously inaccessible annuli can be detected, and corrective actions can be taken by the well operator, before a hazardous event occurs to human life, the environment or property.
The annuli between subsea well casings need to be monitored for pressure to ensure the well is being operated in a safe manner and to satisfy regulatory requirements. Traditionally, only the annulus between the production tubing and production casing string is monitored for pressure for wells drilled through marine wellheads. New regulatory requirements may dictate that all casing annuli be monitored for pressure in the future. The present invention enables pressure to be monitored in the outer annuli of the well casing program without adding any pressure containing penetrations to the well system. This non-intrusive approach to monitoring pressure in the annuli preserves the pressure integrity of the well and maximizes the safety of the well.